Although large amounts of soda ash (anhydrous sodium carbonate) are produced by the well known Solvay Process, an increasing amount is obtained from natural trona, large deposits of which are found in the vicinity of Green River, Wyo. at depths varying from 800 to 1800 feet. The trona beds are generally sandwiched between, the sometimes contaminated with stratifications of shale. The trona consists mainly of sodium sesquicarbonate (Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O), and typically contains about 4 to 6 percent of insoluble matter consisting largely of shale, but also containing calcareous minerals such as calcite (CaCO.sub.3) and shortite (Na.sub.2 CO.sub.3.2CaCO.sub.3).
In the production of sodium carbonate from natural trona, it is common procedure to calcine the trona to form crude sodium carbonate: EQU 2Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O.fwdarw.3Na.sub.2 CO.sub.3 +5H.sub.2 O+CO.sub.2
the sodium carbonate thus obtained is dissolved in water or process liquors, usually dilute sodium carbonate liquors. In that operation, calcerous minerals also dissolve to some extent, so that the resulting sodium carbonate solution may contain up to about 50 parts per million (ppm) of dissolved calcium salts, expressed as calcium.
During subsequent operations, super-solubilized calcium salts tend to deposit from the process liquors to form hard, tenaceously adhering scale on the surfaces of processing equipment, such as heat exchangers, pumps and lines, particularly in zones of high turbulence and high temperature. These scale deposits predominantly consist of pirssonite (Na.sub.2 CO.sub.3.CaCO.sub.3.2H.sub.2 O) in the form of a compact, adherent crystalline mass which may vary in thickness from a thin film to a layer of 1/4" thickness or more. These deposits not only restrict fluid flow and heat transfer, but also cause pitting, corrosion and breakdown of moving parts. Because of their hardness and adherence, these deposits are difficult to remove by mechanical means.
Since the source of the calcium is the insoluble matter contained in the trona, it has been proposed to quickly separate the sodium carbonate solution from the insoluble matter, prior to any attempt to remove the solubilized calcium from the solution. This, apparently, has not met with much success and has not obviated the need for periodic mechanical and/or chemical removal of scale from the equipment entailing costly down time.